Helicopter survey should aid groundwater planning

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A helicopter carrying an Airborne Electromagnetic system takes off in November from the Orland airport to survey layers beneath the soil in Butte and Glenn counties to help with groundwater management. (Butte County Department of Water and Resource Conservation)

Butte County may soon have a better idea of what lies beneath its surface, thanks in part to the Kingdom of Denmark.

Starting in late November, a helicopter took off for several days from the Orland airport to fly a pattern over an area between Chico and Orland, and southeast into Butte Valley.

Dangling beneath the helicopter was a hoop loaded with devices that created a weak magnetic field and instruments that measured how that interacted with layers beneath the soil.

Christina Buck with the Butte County Department of Water and Resource Conservation explained that underground there are layers of sands and gravels that hold water, divided by layers of clay and silt that block water passage to different degrees.

The different materials send different signals, though the picture gets blurrier down toward 1,200 feet, she said.

There’s a problem that while the signal received identified where the layers were, it didn’t identify if they were the water-bearing aquifers, or the water-blocking aquitards. So the flight patterns were planned to pass near monitoring wells where county officials already know what layers are where. That provided a legend for interpreting the data from the helicopter survey, and the knowledge of what exists near the wells could be expanded over a much broader area.

It’s important because local water agencies have been charged by a recent state law with developing groundwater management plans that prevent adverse effects on water supply and water quality. Knowing where the groundwater is and how it moves below the surface would result in better plans.

For example, there’s a major aquifer beneath Butte County and other Sacramento Valley counties called the Lower Tuscan. In some places, it seems to be isolated from other aquifers by impenetrable layers of clay. In other places, it seems “leaky,” according to the county webpage on the project. The helicopter data might show differences in the clay layers that would explain that.

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There’s also a question of how water gets into the Lower Tuscan in the process called recharge. It’s believed areas around Butte Valley might play a role. The look into what’s underground might give insight whether that’s the case and how it might work.

The survey process is known as Airborne Electromagnetic (AEM). The helicopter flew about 500 miles in total, 50 to 100 feet above the ground. The survey lines were about 500 yards apart in the critical areas, back and forth, “similar to mowing a really big lawn,” according to the county webpage.

“The AEM survey essentially takes an MRI of the ground to help us better understand the structure of sand and clay materials underground and the water system to implement the Sustainable Groundwater Management Act,” said Paul Gosselin, director of Department of Water and Resource Conservation in a press release.

AEM has typically been used to locate potential mineral deposits. But Denmark, which relies entirely on groundwater, according to Buck, adapted the technology to map its aquifers and plan to manage them. “They were where we are now about 20 years ago,” she said.

Steve Schoonover is the city editor of the Enterprise-Record and Oroville Mercury-Register. A resident of Chico since 1963 and a 1975 graduate of Chico State University, he has been with the E-R since 1980.